In this coastal town, rising seas exacerbate high-tide flooding
SAVANNAH, Ga. — The Fort Pulaski tide gauge sits on a small, narrow pier just 15 miles east of Savannah, Georgia. It is one of more than a hundred stations across the country that track high and low tides, temperatures, wind speeds, air pressure — and the rising seas.
For 90 years, this station has returned a steady stream of data to scientists, locals and ship captains, helping them track the rhythms of the ocean and avoid perilously low tides.
It has also shown a dangerous trend: Since 2010, the sea level at the Fort Pulaski gauge has risen by more than 7 inches, one of the fastest rates in the country, according to a Washington Post analysis of National Oceanic and Atmospheric Administration data for 127 tide gauges.
Similar spikes are affecting the entire U.S. Southeast — showing a glimpse of our climate future.
The United States has been recording tides for around two centuries. In the early 1800s, tide gauges were simple vertical staffs placed in the water, with individual monitors checking them every hour and recording data by hand. Later, an American inventor developed a tide gauge that recorded the levels without human intervention — a float rested on the top of the water, connected to a pencil above that traced out patterns on a recording sheet.
Now, the technology is much more sophisticated. NOAA’s tide gauges are outfitted with backup sensors, satellite communications and a version of GPS that allows precise measurement of the gauge height from a network of satellites around the globe.
That data has been critical for navigating harbors and channels, predicting the best time for fishing, and spurring trade. But today, it has taken on an even more urgent role — helping to track the seas rapidly rising due to human-driven climate change. And in some spots, like the area around Fort Pulaski, that sea level rise has accelerated.
The range of tides at Fort Pulaski rise and withdraw throughout the year, driven by the pull of the moon and sun as well as the weather.
When the tides rise high enough, they surge onto the streets of nearby Savannah.
Rising seas have made high-tide floods more frequent since the station was established in the 1930s.
Overall, sea levels here rose 7.3 inches between 2010 and 2023; in the previous 30 years, the ocean rose about 3.7 inches.
And the deluge stretches all across the South and the Gulf Coast; over the past 14 years, sea levels in the U.S. South have risen twice as fast as the global average.
“It’s really the hot spot,” said Ben Hamlington, a research scientist who leads NASA’s sea level change team.
The speed has worried scientists. “What we’ve seen over the past decade, decade and a half in the Gulf of Mexico is faster than most climate projections,” said Chris Piecuch, an assistant scientist at the Woods Hole Oceanographic Institution. “And that’s what’s causing this big concern.”
The recent surge in sea level is happening across the entire Southeast Atlantic Coast of the United States and in the Gulf of Mexico.
This trend, driven by warming water, changing currents and melting ice, stretches deep into the ocean.
Driving sea level rise
The U.N. Intergovernmental Panel on Climate Change projects that the planet will experience between 1 and 3 feet of sea level rise by the end of this century. But that’s a global average.
Many people think of the ocean like a bathtub — turn on the faucet, and water levels rise everywhere. As the planet warms and ice melts, they assume that the ocean might act in the same way.
Sea level rise, however, is very uneven. Thanks to a complex set of factors, water levels are not rising at the same speed everywhere. And these factors have made the U.S. South particularly vulnerable.
“There are many nonintuitive things that happen” as global sea levels rise, Piecuch said.
Along the U.S. Southeast Atlantic Coast, an area that stretches from Florida to North Carolina, three factors have combined to drive faster sea level rise than most places in America, according to scientists.
Melting ice adds 1.1 inches of sea level rise on the Southeast Atlantic Coast, out of 4.4 inches recorded between 2010 and 2021
First, as ice melts in Greenland or Antarctica, meltwaters spill into the ocean, raising global sea levels everywhere. But, counterintuitively, the coastlines farthest from the ice sheets are hit hardest.
That’s because the ice sheets are gigantic — so gigantic, in fact, that they exert a gravitational pull on the ocean. (The Greenland ice sheet weighs approximately 2.7 quadrillion metric tons, equal to about 450 million Great Pyramids of Giza; the Antarctic ice sheet is 10 times heavier.)
Normally, that enormous weight pulls oceans close to the ice sheets, making sea levels around Greenland and Antarctica higher than they would be otherwise. But as the ice melts, that effect lessens. Sea levels close to the ice sheet fall, and sea levels farther away rise.
Jerry Mitrovica, a professor of earth sciences at Harvard University and one of the leading experts on melting ice sheets and ocean levels, recalls looking at a plot of sea levels near Greenland. “I couldn’t believe it,” he said. “It was just an incredible thing to see around this melting ice sheet that sea level change, at least regionally, is dramatically going down.”
Researchers use satellites to track this effect. Two GRACE Follow-On satellites, launched in 2018 by NASA and the German Research Center for Geosciences, orbit the Earth about 140 miles apart — as they do, subtle changes in Earth’s gravitational pull yank one farther from the other. Those shifts paint a picture of the planet’s gravity, which scientists can use to predict the precise pull of the ice sheets on ocean waters.
Sinking land compounds sea level rise by 0.6 inches
That melting ice is not the only reason seas are encroaching on the coasts. All across the world, land is shifting all the time — sometimes from the steady movement of giant tectonic plates, sometimes from local factors like the pumping of groundwater.
Scientists track these changes with a set of satellites that ping radio waves off the ground and measure how long it takes those waves to return to their sensors. Some time later, the satellite takes another pass over the same area, monitoring how the land has moved closer or farther away.
In the Southeast, for example, the land is sinking by about 0.6 inches per decade — which compounds the impact of rising seas. A small portion of this dip comes from the movement of tectonic plates, which are slowly rebounding from the weight of glaciers that laid atop them tens of thousands of years ago.
But most of it, researchers say, stems from people pumping water from deep underground to drink or to irrigate crops. “As you withdraw water from an aquifer, you’re basically creating a vacuum,” said Leonard Ohenhen, a postdoctoral researcher at the Lamont-Doherty Earth Observatory at Columbia University. “That material can settle down over time.”
Warming oceans and changing currents add 3.1 inches of sea level rise
Finally, scientists look at water temperatures and ocean currents. As the planet warms, 90 percent of the added heat goes into the ocean. But — like sea level rise itself — that temperature rise isn’t spread uniformly across the ocean. Some areas get hotter faster than others, and the surging currents wandering across the globe can carry warm water from one place to the next. Salinity can also change the density of ocean water and, by extension, the sea level.
In general, the effect of temperature on sea level rise is simple: added heat expands ocean waters. Deep ocean waters play a particular role — as they expand, they cause sea levels to rise out in the open ocean. That expanded water then surges onto the coasts, spiking sea levels along beaches and shorelines.
“You have this adjustment where water moves onto the coastal shelves,” said Jacob Steinberg, a research scientist at NOAA’s Geophysical Fluid Dynamics Laboratory.
To measure the increase in sea levels due to rising temperatures, scientists rely on a network of ocean floats that sink underwater, move around with ocean currents and then surface, beaming their data to nearby satellites. One of those networks, Argo, has almost 4,000 floats drifting through the sea.
Scientists can add all these components together, and match them to data from tide gauges and satellite readings of the ocean height. “That gives you confidence that your measurement systems are working properly,” said Sönke Dangendorf, an assistant professor of river coastal science and engineering at Tulane University.
Note: A fourth factor, called the “inverted barometer effect,” which has reduced sea levels along the Southeast Atlantic Coast by an estimated 0.4 inches, is not shown. Unlike the other factors discussed, experts don’t think the inverted barometer effect is linked to human activity.
Ocean warming, scientists say, largely accounts for the rapid acceleration of sea level rise in the Southeast over the past 14 years. But mysteries remain. Only the U.S. Southeast — specifically, the area south of North Carolina’s Cape Hatteras — has experienced faster sea level rise than scientists predicted. In the Northeast, waters are rising, but more in line with projections.
Scientists suspect part of that is because of the Gulf Stream — a long band of warm water that follows the coast up from the equator and then, near Cape Hatteras, turns out into the Atlantic Ocean. The waters of the Gulf Stream and the Gulf of Mexico are warming faster than other parts of the Atlantic, boosting sea levels. “The Gulf of Mexico has warmed exceptionally fast over the past decade and a half,” Piecuch said. “It’s uncontroversial.”
But scientists have puzzled over where all that heat is coming from. “That’s exactly the part where we all passionately disagree,” Dangendorf said. “You could ask three of us, and you’d probably get three different answers.”
Dangendorf believes the heat is coming from winds pushing warm waters from east to west in the Atlantic Ocean, and ocean currents forcing that warmth toward the Atlantic Coast and into the Gulf of Mexico. But other researchers, like Liping Zhang at NOAA’s Geophysical Fluid Dynamics Laboratory, think the heat may also be coming from changes in the Atlantic’s overturning circulation, a massive ocean current belt that moves heat from the equator to the poles.
This rapid rate of acceleration might not last forever. The ocean changes on the timescale of decades or more; the current heat could be part of long-term variations in ocean currents, and not a clear signal of climate change. But the fact that the change is linked to heat — at the same time as the entire ocean is taking on excess heat from global warming — makes some experts suspicious. “This particular mechanism does not immediately suggest it’s just natural variability,” Hamlington said.
‘It’ll be you tomorrow’
For now, sea levels in the Southeast are surging — and they provide an early picture of what most of the United States, and the rest of the world, will experience as oceans rise.
Towns and cities near Fort Pulaski are trying to adapt: shoring up beaches and stormwater systems, and even considering retreating to higher ground as the waters approach.
On Tybee Island — whose population of 4,000 swells to over 100,000 during the summer months — leaders have gotten used to the constant fight against the waves. Five or six times a year, high tides sweep over the one road that connects the island to the mainland, cutting residents off from services. By 2050, scientists estimate, those high tides will happen 70 days a year.
“We have an aging population, and we don’t have health-care facilities here,” said Brian West, the mayor of Tybee Island. “We have to have alternative ways to get people off the island.”
With the help of the U.S. Army Corps of Engineers, the city has built dunes to protect vacation homes and local storefronts from the rising water; many homeowners have also raised their properties high up into the air.
In Savannah, small businesses and city streets are washed in floods even on bright, sunny days — thanks to high tides that surge into the drainage system. The city estimates that it will cost $400 million to update the stormwater infrastructure over the next two decades. So far, it has raised $150 million.
For both cities, it’s a never-ending process; a constant battle against rising waters that sucks up time and money.
On an afternoon in December, Nick Palumbo, a Savannah alderman, peered into a recently excavated crevasse in the center of a road in Midtown. Below, workers were replacing one-foot diameter stormwater pipes with giant five-foot diameter pipes; a process that takes months and millions of dollars. But even those new pipes will only hold back a certain amount of flooding. Then, as the waters rise, the government will have to do something else.
“We’re going to constantly have to adapt, and have this be an enormous part of our budget every year, just to be able to tread water,” Palumbo said. “If we can do that much.”
Other states and cities will soon see the same effects. NASA projections show that in the coming decades, many cities in the Northeast will experience up to 100 more days of high-tide flooding each year. Some researchers think that the Southeast acceleration may be linked to long-term weather patterns in the Atlantic Ocean like the North Atlantic Oscillation. If so, the trend could switch in the coming decades — with areas of the Northeast seeing rapid sea level rise while the trend in the Southeast slows down.
Zhang believes that the Northeast could see a similar acceleration in the coming years. “I can only say that it is very likely,” she said in an email.
Locals on Georgia’s coast say that their experience can be a lesson for the future. “We have to learn to live with the water,” Palumbo said. “The things that we’re going through today — it’s us today, but it’ll be you tomorrow.”
Sarah Kaplan contributed to this report.
About this story
Additional design and development by Emily Wright. Video by Ricky Carioti. Editing by Monica Ulmanu, Juliet Eilperin and Anu Narayanswamy. Additional editing by Dominique Hildebrand, John Farrell and Joseph Moore. Project editing by KC Schaper. Copy editing by Frances Moody.
To analyze and visualize the largest contributors to sea level rise along the coastal United States, The Washington Post relied on data recently published by a group of scientists from U.S. universities and NASA led by Sönke Dangendorf, the David and Jane Flowerree assistant professor of river coastal science and engineering at Tulane University.
They expanded upon prior modeling to estimate how melting glaciers and ice sheets, as well as thermal expansion of ocean water, have contributed to global sea level rise from 1900 to 2021. To estimate the impact of vertical land movement on relative sea levels along the coasts, the researchers incorporated land motion estimates produced by scientists in Europe and the United States.
Dangendorf provided The Post with annual averages for each component of sea level rise at the locations of 127 tide gauges along the Atlantic, Pacific and Gulf coasts, as well as spatial layers breaking down the trends over two periods of time: 1900 to 2009 and 2010 to 2021. To calculate the trends of the main contributors to sea level rise since 2010 along the Gulf and Southeast Atlantic coasts of the United States, The Post applied a linear regression model to the data for the gauges in each of those regions.
To calculate the most recent relative sea level rise at Fort Pulaski from 2010 to 2023, The Post applied a linear regression model to the annual mean sea level figures for that tide gauge.
The Post used annual high-tide flood counts from NOAA for each available tide gauge and grouped the data by decade to track trends. NASA produced the projections for the numbers of high-tide flood days in each area from the 2020s to the 2050s. The Post used the intermediate sea level rise scenario for the high-tide flood projections.
The 3D diagram explaining how the Fort Pulaski tide gauge operates was based on local inspection at the pier and technical information provided by Albert Sanford and Kyle R. Ward at NOAA.
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